1. Technical Field
The present invention relates to wireless communications and, more particularly, to circuitry for switching logic.
2. Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMIS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc., communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via a public switched telephone network (PSTN), via the Internet, and/or via some other wide area network.
Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier stage. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier stage amplifies the RF signals prior to transmission via an antenna.
Typically, the data modulation stage is implemented on a baseband processor chip, while the intermediate frequency (IF) stages and power amplifier stage are implemented on a separate radio processor chip. Historically, radio integrated circuits have been designed using bipolar circuitry, allowing for large signal swings and linear transmitter component behavior. Therefore, many legacy baseband processors employ analog interfaces that communicate analog signals to and from the radio processor.
Within such communication systems and, more generally, within logic circuits and other operational circuits, switching often occurs in response to a signal level reaching a specified threshold. One known problem, however, is that the signal may oscillate about the threshold point causing the circuit output to fluctuate.
FIG. 1 is a functional block diagram of a prior art circuit for rendering a threshold crossing decision. As may be seen, a signal source produces a signal to a threshold comparator that is further coupled to receive a reference voltage Vref. The threshold comparator is operable to provide a first logic output if the signal from the signal source exceeds the reference voltage Vref and a second logic output if the signal does not exceed the reference voltage Vref. As may be seen, fluctuations of the signal received from the signal source due to noise may result in a fluctuating output if the signal and the reference voltage Vref are substantially similar.
In a circuit in which such voltage levels are small, comparators are often used to resolve the polarity of the difference between two voltages and to amplify the very small voltage differences to make a decision. In many cases, noise on the voltages being resolved can cause fluctuations, as shown in FIG. 1. It is desirable, therefore, to make hard decisions that do not toggle due to noise variations. Thus, a hysteresis circuit using a comparator with cross coupled gain devices is sometimes employed wherein positive feedback generates the desired hysteresis to prevent such fluctuating outputs. One problem with these known approaches, however, is that process variations contribute to variations in the effective hysteresis voltage such that a large variation in hysteresis voltages may be experienced. What is needed, therefore, is a hysteresis circuit that is operable to quickly respond to a short or low output impedance situation to reduce the likelihood of damaging the integrated circuit and that reduces any required settle time when transitioning back to a normal mode of operation.